install_venv: refactor out pip_install helper
install_venv: refactor out pip_install helper

We don't need to pass the -E argument since we're already running
inside the virtualenv.

Change-Id: Idd77b9bdc067e2288d0fa694a2bb6769675065cf

zancas/hacking,hyakuhei/cleantox,openstack-dev/hacking,emonty/oslo-hacking,zancas/hacking,hyakuhei/cleantox,emonty/oslo-hacking,openstack-dev/hacking